To reduce the exhaust amount of carbon dioxide gas from automobiles, automobile bodies are being reduced in weight by using high-strength steel sheets. Furthermore, to secure the safety of drivers and passengers, in addition to soft steel sheets, more and more high-strength steel sheets with a maximum tensile strength of 980 MPa or more are becoming to be used for automobile bodies. To further reduce the weight of automobile bodies, the strength of high-strength steel sheets during use has to be higher than before. However, the increase in the strength of steel sheets typically leads to the degradation of material characteristics such as formability (processability). Thus, it is a key to the development of high-strength steel sheets how the strength is increased without the degradation of material characteristics.
Steel sheets that are used for such members are required to have such a performance that the members are unlikely to be damaged even when shocked by collision or the like after steel sheets are molded and attached to automobiles as components. In particular, in order to secure impact resistance in cold areas, low temperature toughness is also demanded to be increased. The low temperature toughness is defined by vTrs (Charpy fraction dislocation temperature), for example. For this reason, the impact resistance of the above steel materials needs to be considered. In addition, high-strength steel sheets are unlikely to be plastically deformed and will occur more easily; thus, toughness is demanded as significant characteristics.
As one of methods for increasing the strength of steel sheets without the degradation in formability, there is a method of baking-hardening using coating-baking. This method increases the strength of automobile members in the following manner: through heat treatment at the time of coating-baking treatment, dissolved C present in a steel sheet concentrates at dislocations formed during molding or is precipitated as carbides. Since hardening is performed after press formation in this method, there is no degradation in press formability due to the increase in strength. Thus, this method is expected to be used for automobile structural members. As an index for evaluation of the baking hardenability, there is known a testing method in which 2% prestrain is imparted at room temperature and then heat treatment is performed at 170° C. for 20 minutes to perform evaluation at the time of retensile testing.
Both the dislocations formed at the time of production and the dislocations formed at the time of press processing contribute to baking-hardening; therefore, the sum of them, which is the dislocation density, and the amount of dissolved C in the steel sheet, are important for the baking hardenability. An example of a steel sheet having excellent baking hardenability while having a large amount of dissolved C is the steel sheet shown in Patent Document 1 or 2. As a steel sheet that secures more excellent baking hardenability, there is known a steel sheet including N in addition to dissolved C and having excellent baking hardenability (Patent Documents 3 and 4).
Although the steel sheets shown in Patent Documents 1 to 4 can secure excellent baking hardenability, these steel sheets are not suitable for production of high-strength steel sheets with a maximum tensile strength of 980 or more that can contribute to high strength of structural members and the reduction in the weight because the base phase structure is a ferrite single phase.
In contrast, being extremely hard, a martensite structure is typically used as a main phase or the second phase in steel sheets having a strength as high as 980 MPa or more to increase the strength.
However, since martensite includes an enormous number of dislocations, it has been difficult to obtain excellent baking hardenability. This is because the dislocation density is high compared to the amount of dissolved C in steel. In general, when the amount of dissolved C is small compared to the dislocation density in a steel sheet, the baking hardenability is degraded. Accordingly, when soft steel that does not include many dislocations and steel of a martensite single phase are compared with each other, if the amount of dissolved C is the same, the baking hardenability of the martensite single phase is more degraded.
Therefore, as steel sheets that were attempted to secure more excellent baking hardenability, there are known steel sheets having higher strength by adding an element(s) such as Cu, Mo, W, and/or the like to steel and precipitating carbides of these elements at the time of baking-coating (Patent Documents 5 and 6). However, these steel sheets do not have high economic efficiency because the addition of expensive elements is necessary. In addition, even though carbides of these elements are used, it has been still difficult to secure the strength of 980 MPa or more.
Meanwhile, as for a method for increasing the toughness of a high-strength steel sheet, for example, Patent Document 7 discloses a method for producing such a steel sheet. There is known a method in which the aspect ratio of a martensite phase is adjusted the martensite phase is used as a main phase (Patent Document 7).
In general, it is known that the aspect ratio of martensite depends on the aspect ratio of austenite grains before transformation. That is, martensite having a high aspect ratio means martensite transformed from unrecrystallized austenite (austenite that is extended by rolling), and martensite having a low aspect ratio means martensite transformed from recrystallized austenite.
From the above description, in order to reduce the aspect ratio of the steel sheet of Patent Document 7, it is necessary to recrystallize austenite; in addition, in order to recrystallize austenite, it is necessary to increase the temperature of final rolling. Accordingly, the grain size of austenite and also the grain size of martensite have tended to be large. In general, grain refining is known to be effective to increase toughness. A reduction in the aspect ratio can reduce factors that degrade toughness due to the shape, but is accompanied the degradation of toughness due to coarse crystal grains; therefore, there is a limit on the increase in toughness. In addition, Patent Document 7 mentions nothing about the baking hardenability that a study of the present application has focused on, and Patent Document 7 hardly secures sufficient baking hardenability.
Furthermore, Patent Document 8 discloses that it is possible to increase the strength and low temperature toughness by finely precipitating carbides in ferrite having an average grain size of 5 to 10 μm. By precipitating dissolved C in steel as carbides including Ti and the like, the strength of the steel sheet is increased, so that it is considered that the amount of dissolved C in steel is small and excellent baking hardenability is unlikely to be obtained.
In this manner, it has been difficult for a high-strength steel sheet with 980 MPa or more to have both excellent baking hardenability and excellent low temperature toughness.